(a) Field of the Invention
The present invention relates to an organic light emitting diode display and a method for manufacturing the same.
(b) Description of the Related Art
Recent trends toward light and thin personal computers and television sets require light and thin display devices. Flat panel displays such as a liquid crystal display (LCD) that can satisfy such requirement are thus becoming increasingly common substitutes for conventional cathode ray tubes (CRTs).
However, because the LCD is a passive display device, a backlight is needed as a light source. Associated with the fact that LCD is a passive display device are various problems such as slow response time and narrow viewing angle.
Among the flat panel displays, organic light emitting diode (OLED) display has recently been the most promising as a display device for solving these problems.
An OLED display is a self emissive display device, and includes two electrodes and an organic light emitting layer interposed between the two electrodes. One of the two electrodes injects holes and the other injects electrons into the light emitting layer. The injected electrons and holes are combined to form excitons and the excitons emit light as discharge energy upon changing their energy levels.
Some advantages of the OLED display include its low power consumption, fast response time, wide viewing angle, and high contrast ratio.
OLED displays are divided into passive matrix OLED displays and active matrix OLED displays according to how they are driven.
The active matrix OLED display includes a plurality of switching thin film transistors connected to signal lines that cross each other, a plurality of driving thin film transistors connected to the switching thin film transistors and driving voltage lines, and a plurality of emitting portions connected to the driving thin film transistors.
The semiconductor of the TFT is made of amorphous silicon and polycrystalline silicon according to the crystal structure of the silicon.
Amorphous silicon is used in displays utilizing glass having a low melting point, since an amorphous silicon film can be fabricated at a low temperature. However, amorphous silicon film may not be well suited for application to a high quality driving circuit in display panels because it has low carrier mobility. Since polycrystalline silicon has prominent electric field effect mobility and high frequency operation, high quality driving circuits often use polycrystalline silicon. However, the TFT of polycrystalline silicon has difficulty in the manufacturing process and disadvantages such as a large leak current. Particularly, when etching the semiconductor, the surface of the semiconductor may be damaged such that the current leakage may be increased.